Automatic control for locked oscillators



June 12, 1945.v w. A. Mccool.

V AUTOMATIC CONTROL FOR LOCKED OSCILLATORS Filed nay 9, 1942 s sheets-sheet 1 .LUCK/N6 CHARACTER/$7@ w C wn fm. a

jme/who@ Willtaln A. -Mc Cool vJune 12, 1945.

ocx/Ns mrow/vcr fio FLA'I -6077/ 7065s w. A. MccooL AUTOMATIC CONTROL E'OR LOCKED OSCILLATOHS Filed nay s, 1942 :ELE-: 4

3' sheets-sheet 2 William Ale Cool FREQUENCY June 12, 1945. w. A. MccooLl 5 2,377,394 AUTOMATIQ CONTROL Fon LocKED oscILLAToRs V l Filed I Iay 9, I1942 I s sheets-sheet 3 a sj 'www www;

' gwwm lPl/zcun A. lcCool Patented June 12, 1945 UNITED STATES PATENT OFFICE AUTOMATIC CONTROL FOR LOCKED OSCILLATORS William A.Mcco01,washingion,n. C. Application May 9, 1942, serial Nq. 442,340

(ci. 25o- 36) (Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 0. G. 757) 6 Claims.

This invention relates to a method and means for increasing the stability of multivibrators and more particularly to amethod and means which render the multivibrator output frequency independent of variations of locking voltage amplitude over a wide range of such variations. While directed particularly to multivibrators, the invention is applicable to other types of oscillators.

A multivibrator consists essentially of a twostage resistance coupled amplifier whose output is capacitatively coupled to its input. Oscillations are sustained in this arrangement because a 180 phase shift is produced in each tube thereby setting up the exact phase relationship for complete regeneration. The natural frequency of oscillation is' determined primarily by the coupling capacitorsand the grid leak resistances and it is also influenced to a much` lesser degree by the tube characteristics, the electrode voltages and the remaining circuit constants.

Multivibrators have been used-for years in frequency measuring applications where successive division of the standard frequencyis necessary. Their highly distorted wave form is especially useful for harmonic generation. In recent years they' have been employed intelevision for the generation of locked sweep frequencies, and are well suited for other applications inhighly specialized types of radio equipment. When used as frequency dividers the frequency of oscillation is a sub-multiple of the controlling or locking voltage'frequency. Division ratios of 50 to 1 `have been attained but a ratio of 10 to 1 has heretofore been a sound practical limit. The oscillation frequency limits are consistent with those of resistance coupled amplifiers. l

AThe most common source of trouble heretofore encountered in the use of multi-vibrators is complete failure or variation in locking voltage. Since lthe division ratio is a function of locking voltage amplitude, as will be shown later, it follows that a failure or a variation in the locking voltage will usually produce a change in division ratio. This is of course an undesirable effect. Insome frequency measuring applications where many frequencies are derived from a standard source such as a crystal, multivibrators are used in cascade. For example, 10,000 cycles, 1,000

cycles, 100 cycles-and 10 cycles may be derived froma 100 kc. crystal and locked with it. In this case the crystal `controls a 10,000 cycle multivibrator'which in turn controls a 1000 cycle multivibrator and` so on. If any one unit vloses control the remaining units on the low frequency side usually lock together in some random .har-

monic relationship. Such failures are not uncommon and are highly undesirable. f

Other types of oscillators are also subject to defects of a similar nature. Such oscillators possess locking characteristics 'similar to those of multivibrators. In 'such cases, when the locking voltage amplitude variation exceeds a certain range the oscillator will take up some random frequency of oscillation. In certain applications,

the use of a type of locked oscillator other than a multivibrator becomes desirable. The invention may readily be used in controlling the locking of such oscillators. 1-

It is therefore an object of this invention to provide a method and means for rendering the output frequency of a multivibrator or other oscillator independent of variations in locking voltage amplitude over'a widerange of such variation and for rendering cessation of operation in the complete absence of locking volt/age.

It is a further object of this invention to control the stability of a multivibrator or other oscillator by rendering the `locking voltage amplitude directly proportional to the plate supply voltage.

It is another object of this invention to control the stability of a multivibrator or other oscillator by varying the amplitude of the plate supply voltage as a linear function of the lockingr voltageamplitude.

It is still another object of thisinvention to provide a method and means for deriving the plate supply voltage of a multivibrator or other oscillator from the source of the locking voltage applied thereto.

Other objects, of this invention will become apparent from a careful consideration of the following description when takenl together with the accompanying drawings in which:

Fig. 1 is a graph showing the relationship between Vthe multivibrator output frequency and locking voltage amplitude;

Fig. 2 is a graph showing the relationship between locking voltage and plate supply voltage for a particular division ratio;

Fig. 3 is a graph showing the natural frequency characteristic of a multivibrator;

Fig. 4 is a graph showing a desired curve of relationship betweenA locking voltage amplitude and plate supply amplitude, and a diode characteristic curve; Y i

Fig. 5 is a circuit diagram of a portion of a circuit from which is derived according to the invention, multivibrator 'locking' voltage and plate supply voltage;

Referring now to Fig. 1 in which is shown-"1 variations in multivibrator output;frequency:for various conditions of locking voltage and plate`v supply voltage, it will be noted that with plate voltage constant the output frequency I of" thev multivibrator varies with variations in'flockingvoltage amplitude in a series of discrete steps, the frequency of each step bearing anvintegral proportionality to the locking voltage frequency.

It should be noted that the maximumy frequencyl limit is the frequency of the lockingvoltage and the' minimum-frequency is- -thenatura1irequency of the multivibrator. Fig; lshows-fourcurves eachV drawn` for a'diiTerentva-luefof`v plate supply voltage. Thesecurves aremarked-Em, Em, Em and.Ee4. Thus, from thisfigure it will be; seen that not only vdoes the; multivibrator-output frequency vary withvariations inlocking voltage amplitude through a series-oi discrete steps, each frequency bearing an integral ratio tov the-loch;-A ingv voltage frequency; but that the point at whichA each of these steps occurs with respectI to locking voltagey amplitude variesgwithf plate supply voltage amplitude.`

Fig. 2A shows-for a givenldivisionratio -thepermissible variation inglocking-'jvoltage-for various values-fof, plate suppl-y voltage.v The` maximum and'minimum loclring voltage amplitudes used in this graph, aref taken'V directlyfromfaf family fof curvessimilar tothat shown inl -Figr l forl a division ratioof 10 tov l. This' plotfresultsfin two spa-cedfandfalmost parallel curveslwliich are linear over most of thefplatevoltage range; `For any particularv value oit-'plate supply voltage-the` ultimate distance between theetwofcurvesdefnesthe ,of stable operation. It can be seen from the curves that this minimum may be extended by a choice-of circuit .constants which will determine the same natural frequency (equal to the operatingfrequeney) at a lower plate supply voltage.

`'Thepractical'limit of minimum plate voltage is 7410.10 vol-tsgthe rminimum for stable operation of the multi-vibrator is somewhat greater, the extent being determined principally by the division'V ratio.l

Dueto thelinearity of each curve and the similarity of their respective slopes the curves of operation.v Obviously,l

limits of locking voltage amplitude. As the divi- 'f sion ratio is increasedthese'curveswill'fall nearer each' other indicating-that the-'permissible -limitsA ofA the lockingvoltage are' becomingmorerestricted. ThusA the`v critical adjustment of` the conventional multivibrator is` graphically indicatedf In Fig. 2 it will observed' that the maximum and minimum curves convergeat zero locking voltage and a low valueof platesupply voltage. The point of convergence de'nesthe plate voltage at which the naturalfrequencyis coincident with the initial multivibratorI controlled frequency.`

The eilect o fplate supplyvoltage'on the-natural frequency ofA the@ multivibratorisi shown Aby the curve in Fig; 31 v This invention is based upon therelationship between plate supply voltage and locking voltage amplitudes illustrated in Fig. 2; It'eontemplates making thev lock-ing voltage' amplitude directly proportional to the-plate supply voltage so that any/change of' the latterv will' procluci-e'fay linear variation of the `formeri Theratio of -tl'iesev )1t" ages` is. adjusted -so thatithe locking voltage curve falls m-idwayfbetween the maximum and minimumf curves. Such aA curve is illustrated" in Fig. 4, the locking voltage curve being designated EL; Fulillment off'thisconditionfresults in severaldistinct'and striking advantages; ilrst, the rangev of`1ocking voltage amplitudeior'a given Fig.4 Ltido notl indicate ama-ximum rpointofl stable however; aflmaximum pointy exists` at a' higher plate supply' voltage which is in excessgoflthe'highest practical lim-it.

Fig. 5 illustrates one practical means lby -which the inter-relationshpy of' locking-v voltage and plate Supplyvoltage necessary'/ to f produce the resultsof-Fig. 4' can'be obtained. In thiscircuit a voltage Ein having-the-desired locking vfrequency is impressed' acrossa diode l0 by means `of`a transformer/T acrosst he-secondary of which is connected'a variable resistancel R1. The-Dl C. outputjof` the diodemay-'betaken` acrossA the capacitor Ci. In suchl a circuit theiDfC.' output volta-ge'is directlyi proportional tothe A. C; voltage'applied if theloadresistanceeisimuch greater than the-plateresistance and iii the time constant ot'RandfCl' isi-much greaten'than'the frequency oftheA. C2 voltage. 'I-hese'cor-idi'tionsare quite feasible in practical application--- I'h'evoltagel divider comprising the variable resistancef R1' connected across the signal source provides locking voltage 'havingf any" desired am'- plitude; FromFig. 4'; which `also shows the diode characteristic' it' isf obvious that all three parametersfEi; EL and'Eh arein linear relationship: Thusthedesired` relationship of direct proportionality betweenthe locking; voltage amplitude andthe plate supplyvoltage'is attained.

'l'herejremainsy the problem'of the successful applicationv to the zvnultivibrator of the voltages available' from the diode circuit. The injection of'th'e'locking voltage lcan be accomplished by several diierent' arrangements, one of which is illustratedin Fig.v '7; The DL C. load' presented by the multivibratorI can be substituted directly forthefdiode load if 'the former-,is essentially constant-over aj complete cycle and-over the plate voltage`A range;

Fig. 6 illustrates themultivibrator'load characteristic which, as can beseen, is practically linear. From a study ofi' the1 conventional multivibrator characteristics' ity canvbe seen that the total plate current drain is constant over a cycle, sinceffthe'tubes operate 180out o fphase. Aside fromthe phase'idispla-cement the -plate current'- time chara'cteristicof each'y tube-is exactlyalik'e; When the circuit constants',`1 are symmetrical 'the actual value ofthe load then isI almostv equal to the plate loadresistancej oone tube if' thelatter is considerably`A larger/ than the normal plate resistance.

Fig; 7shows th'efcomplete circuit of a stabilized multivibratorl embodying-f the invention. The

asvasee 3 multivibrator comprises two' triodes I I land I2, the .plate of each being coupled to the grid `of the other tube through symmetrically chosen capacitors C3 and Ci. 'I'he grid resistors R3 and R4 are symmetrical as are the plate resistors Rsand Re which are selected to have a value considerably larger than that of the plate resistance. Input leads I9 and 20 are connected across thedio-de used for the diode and the following values for 'l thevarious elements of the circuit will be found satisfactory. 01:16h, C2=.1.f, C3 andV C4 .0021;if, R1 andRz 10,000 ohms, Rs and R4 360,000 ohms, and R5 and Re 100,000 ohms. i

Fig. 8 shows the operating characteristico tained from a system constructed in accordance with Fig. 7 and employing the values listed above. 'Ihe maximum and minimum curves indicate the limits of the locking voltage amplitudeagainst the plate supply voltage. t shows the operating characteristic for the best xed adjustment of locking voltage. The latter curve is also the diode characteristic with the multivibrator load. Itshould be noted that this curve is not quite linear. This isdue to the tact that even though the plate load resistors R5 and Re represent the dominant portion of the diode load, the plate resistance of a triode increases rapidly with decreasing plate voltages of a small order. Since the D. C. load in any half-cycle of the operating frequency consists of the plate load resistance in series with the plate resistance of one tube, there is a substantial net increase when the latter increases to values comparable to the former.

It will be noted that the curves of Fig. 8 satisfy the requirements of Fig. 4 and that the conventional multivibrator characteristic as illustrated in Fig. 2 has not been altered in the least. The

optimum curve approaches linearity and interz' sects the maximum curve at a low plate voltage of slightly more than 20 volts. The maximum and minimum curves converge at Zero locking volts and 20 plate volts. Furthermore, the multivibrator is inoperative in the absence of locking voltage. It is not dependent on an external plate voltage supply, and drifting tube characteristics are reduced to a point of negligible eect with proper design. y

However, there are minor disadvantages which can be minimized but not completely eliminated. First, a small amount of power is required from the signal source to operate the system. A system constructed using the values listed above requires approximately 100 milliwatts to maintain an effective plate voltage of 100 volts. This is not excessive but under certain circumstances will present impedance matching problems. In this case the effective diode input impedance is approximately 50,000 ohms. In general it may be said that the input impedance is equal to V2 the total static load of one multivibrator tube with proper design, When locking VJfrequencies of 20 kc. or less are used, a 6J5 type tube may be used as an ampliner in conjunction with a The optimum curve i 2 to 1 step-up transformer if no selectivity is desired. Such a combination is shown in Fig. 9 in which tube I 3 feeds into transformer T which is coupled to the'diode I0. It must be remembered that a low resistance D. C. return for the diode is'always necessary. The unstable region between zero and minimum plate voltage may at times constitute another disadvantage. In a system employing the above listed circuit values this region extends from zero to 20 volts of signal voltage. Considering the wide range of operation available above the minimum and the improvement over pre-existing types, this is normally not objectionable. `IIowever, where the ultimate in reliability is desired, this defect may readily be eliminated by a conventional relay deviceemployed 'to prevent operation of the multivibrator below a predetermined minimum locking voltage which, in thisy case, could be chosen as 30 volts input to the diode;` A typical relay which has been satisfactorily tested for this purpose is of an electronic nature employing a thyratron for control. This arrangement has been found to lbe'very reliable. In most cases, however,` such an elaboration of the stabilized multivibrator is not warranted. i 1

-The vastly increased stability provided by the invention makes possible the practical accomplishment of higher division ratios. By employing the invention ratios as high as 20 to l may be attained with a corresponding increase in stability over the conventional multivibrator of the same frequency. Naturally, all the factors affecting instability will become 'more pronounced at such ratios and the operating rangeof input voltage will be materially decreased.

Fig. 9 also shows the application of the invention to oscillators in general, the oscillator being indicated by the reference character I5. The circuit connections between the diode and the oscillator are similar to those of Fig. 7.

The invention should not be considered restricted to the embodiments disclosed herein, since many other arangements coming within the scope of the invention as defined by the appended claims may be employed.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

I claim:

1. Means for stably locking the output frequency of a free-running relaxation oscillator at a fixed integral ratio with respect to the frequency of a locking voltage for a wide range of amplitude variation of said locking voltage, by establishing and maintaining a ixed amplitude ratio between the plate voltage applied thereto and said locking voltage, comprising a locking circuit for said oscillator, a rectifier and filter, means coupling an alternating current voltage having the desired locking frequency across the input to said rectifier, means applying the output of said rectier through said filter and said locking circuit to said oscillator as plate voltage,

means applying said alternating current voltage through said locking circuit to said oscillator as locking voltage and means adjusting the ratio of said plate voltage to said locking voltage applied through said locking circuit.

2. Means for stably locking the output frequeny of a free-running relaxation oscillator at a nxed integral ratio with respect to the frequency of a locking voltage for a wide range of amplitude'. variation 'i ot. saidlockingf-voltage, by establishing and` maintaining; a\xed amplitude ratio vbetween:the:y plate.I voltage;- applied.- thereto andsaid locking vo1tage, comprising a-,loeking circuit forv saidl .oscillatorL .aV rectifier. and; filter; means impressing an alternating current .voltage havingA the. desired locking frequency across lthe input .to said frectienymeans applying; the out'- putxof said rectierthrough said Iillter.V and said locking circuit to: said oscillatoras plate volt'- age, and. means applyingfsaid alternating current voltageA through said; locking circuit to-said oscillator` as locking voltage;

3. Means forr stably locking the; output..fre' quency vof. a free-running relaxation oscillator at a-iixed integralratio withrespect to the frequency of .a locking voltage for a widerange of amplitudevariation of. said locking voltage', by, establishing and maintaining a fixed' amplitude ratio between the plate voltage applied thereto and'said locking voltage, comprisingra source of alternating. current voltage having the desired locking frequency, means deriving from said alternating current voltage a direct current voltage having an amplitude proportionalto the amplitude of said alternating current voltage, means. ltering said direct current voltage andy applying it to said oscillator' as plate voltage'and means applying said alternating current voltagefto said oscillator as locking voltage.-

4. A method of stably locking the output frequency of a'. free-running relaxation oscillatorzat a. xed integral ratiowithrespect tothe frequency` of` a; locking voltage for.l a wide range of amplitude variation of said locking voltage by remesa establishing z and .l maintaining i a: xedjamplitude ratio. between the; plate: voltage:V applied",` thereto and'. Said. lockingfyoltage," which'. comprisesfrderiving from acommon source. an alternating.' curi rent voltage and a..direct current voltage, apply ing-said direct current voltage to saidoscillator as plate voltage and applying said alternating current voltage-to said oscillator as locking volt'- age.

5. A method of stably locking-the output'ffrequency of a; free-runningrelaxation'oscillator'at a xed: integral ratiov with respect to the frequency of alockingvoltage for'a'widerange of amplitude variation of saidv locking" voltage by establishing vand maintaining a lxedi` amplitude ratio ybetween the platevoltage applied thereto and said lockingvoltage, which comprises rectifying and filtering an'alternating'current'voltage havingthe desired lockingv frequency; applying saidv alternating current voltage to said voscillator as locking'voltage and fapplyingsaid rectified and lteredvoltage to said oscillator asplate voltage;

6. A method of.' stably locking the output irequency` of a free-running relaxation oscillator at axed integral ratio with respect. to the frequency of a locking voltage for'a wide range of' amplitude variation ofA said locking voltage, which comprises varying the amplitude of the plate `supply voltage of said oscillatorasa linear function of the Vamplitude of saicl'locking voltage over said range of variation and applying said, locking voltage to said free-running oscillator" to determine the' frequencyl thereof. 

